Definition 2 (d-Separation [1]}) 
Consider a causal graph \(\mathcal {G}\) . \(\mathbf {X}\) , \(\mathbf {Y}\)  and \(\mathbf {W}\)  are disjoint sets of attributes. \(\mathbf {X}\)  and \(\mathbf {Y}\)  are d-separated by \(\mathbf {W}\)  in \(\mathcal {G}\) , if and only if \(\mathbf {W}\)  blocks all paths from every node in \(\mathbf {X}\)  to every node in \(\mathbf {Y}\) . A path \(p\)  is said to be blocked by \(\mathbf {W}\)  if and only if:
1) \(p\)  contains a chain \(i\rightarrow m\rightarrow j\)  or a fork \(i\leftarrow m \rightarrow j\)  such that the middle node \(m\)  is in \(\mathbf {W}\) , or 2) \(p\)  contains an collider \(i\rightarrow m\leftarrow j\)  such that the middle node \(m\)  is not in \(\mathbf {W}\)  and no descendant of \(m\)  is in \(\mathbf {W}\) .

